1. Field of the Invention
This invention relates generally to hydrostatic shock simulation systems and more particularly to apparatus for simulating the shock effects on barge-mounted equipment of an underwater weapon detonation.
2. Description of the Related Art
Perhaps the most demanding requirement of a naval surface environment is the effects of underwater explosions and concussions. Major underwater explosions transmit impulses through nearly incompressible water, causing substantial shock impulses of up to 50 g or more to the vessel. The shipboard equipment on Navy vessels must be designed to withstand the significant shock effects of underwater weapon detonations during wartime. The design of such shipboard equipment requires shock testing to verify design efficacy and the Navy has developed and promulgated the shock test specification MIL-S-901D (17 Mar. 1989) setting forth the requirements for qualifying the design of shipboard equipment. MIL-S-901D also classifies shipboard equipment as a “principle unit” that is directly supported by the ship structure, a “subsidiary component” that is a major part of and affects the shock response of a principle unit, or a “subassembly” that is a part of but does not materially affect the shock response of either a principle unit or a subsidiary component. This Military Specification defines a “heavyweight” test category that mandates shock testing “on a standard or large floating shock platform” that herein is denominated a “test barge.” Acceptable testing for heavyweight shocks is quite challenging, usually requiring actual detonation of explosive devices near a floating barge containing the principle unit equipment. Some of the most devastating impulses are created by the aftershock concussions (secondary bubble pulses), which result in a decaying wave of compression and rarefaction that transmits even greater shock energy to the unit under test. In one test scenario, a specific explosive charge is detonated 24 feet below the surface and electronic equipment mounted on the hull of a floating barge is subjected to the impulse. The range of shock varies, but can approach 50 g. According to MIL-S-901D, the lighter test categories may be performed in the laboratory by using various shock machines but, to properly test for the effects of secondary bubble pulses, the heavyweight tests must be performed on a test barge subjected to an actual underwater detonation, which is intrinsically hazardous.
As used herein and in the art, a barge generally denominates a flat-bottom boat for carrying heavy loads in rivers or canals. The flat bottom and general absence of provisions for withstanding large wave-induced stresses distinguishes the barge from other marine vessels such as those intended for operation on the high seas.
The two conventional land-based Navy shock tests are the Lightweight Shock Machine and Medium Weight Shock Machine, adapted for testing no more than 550 lbs and 7,400 lbs of mounted equipment, respectively, according to MIL-S-109D. These two hammer tests do not properly stress equipment that is resiliently mounted because they provide neither a sufficient platform displacement nor a proper frequency response in the lower end of the shock response spectrum (SRS). Adding a spring-deck apparatus to the Medium Weight Shock Machine can adjust the SRS but does not achieve sufficient platform displacement. A high platform displacement at low frequencies with resonant characteristics similar to a ship deck is necessary to properly stress equipment protected with shock isolators because only this condition can properly bottom-out the shock isolators. When the isolators are bottomed-out, the equipment can be subjected to a more severe shock than seen without isolation (i.e., “hard-mounted”). The heavyweight shock test on a barge subjected to underwater explosive detonation was until now the only test known to properly stress principle unit equipment resiliently mounted on ship decks. Such a barge test is considerably more expensive and hazardous and requires more lead time than a land-based laboratory test. The heavyweight barge test ensures that the principle unit and other items are subjected to a shock wave representing a realistic simulation of actual operational conditions aboard a Navy warship. Until now, such a shock wave could not be realistically simulated in the land-based laboratory.
Several practitioners in the art have proposed improved shock simulation systems for laboratory testing and qualification of electronic equipment. For example, in U.S. Pat. No. 6,182,495 B1, Hansen discloses a test machine for simulating shock-wave induced motion adapted to be carried on marine vessels during sea travel. The equipment under test is mounted on the table of a test machine through which shock-wave induced motion to be experienced by such equipment during the sea travel is simulated and applied to the equipment carrying table by means of coil springs. Such simulated motion is initiated from a position of the table to which it is displaced and from which it is selectively released through a latch device. However, Hansen neither considers nor suggests solutions to the MIL-S-901D heavyweight secondary bubble pulse simulation problem. In a recent publication, Brooks et al. [Rick Brooks and Jack Berenholz, “Part 1 Case Study: Testing High-Powered Signal Processing Systems for Harsh Environments,” COTS Journal, October 2002, pp. 40–45] proposes using a finite-element analysis to mathematically analyze the system, the results of which are then validated with actual or simulated barge tests requiring much lower shock levels that can be performed in the laboratory. Only after testing and validating the equipment design using this approach do Brooks et al. complete the equipment fabrication and subject the final equipment to the actual MIL-S-901D testing. Brooks et al. are primarily concerned with validating the equipment design before final shock testing to avoid the time and expense of a complete redesign cycle in response to a shock test failure and neither suggest nor consider any solution to the MIL-S-901D heavyweight secondary bubble pulse simulation problem.
A traditional underwater detonation induces a complex dynamic ship response that results from several different loading mechanisms. The prominent loading events are the initial shockwave and one or more ensuing bubble pulses. Secondary loadings include bulk and local cavitation closure, reflected pulses from nearby boundaries, and flow effects. The significance of these various loading mechanisms vary greatly depending on the attack geometry.
There is a clearly-felt need in the art for testing methods that employ non-explosive energy sources that provide a useful alternative to conventional barge shock testing methods. A desirable solution is a low-cost and environmentally-friendly non-explosive testing method capable of replicating the dynamic response induced by the complex loading mechanisms presented by a variety of underwater explosive attack scenarios. These unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.